1. Field of the Invention
The invention relates to a method for cleaning the surfaces of medical implants. More specifically, the invention provides a method for cleaning medical implants provided with a porous coating to reduce debris particulates in the coating and remove loosely adherent coating particles, thereby providing a more biocompatible medical implant.
2. Description of the Related Art
It is often desirable to provide medical implants with a roughened (i.e. grit-blasted, peened, etc.) surface or porous coating that will promote bone and tissue integration or ingrowth into surface pores thereby enhancing the fixation of the medical implant in a recipient's body. A porous coating may consist of sintered beads, diffusion bonded wire-mesh, plasma-sprayed metallic powders and the like. These roughened surfaces or porous coatings may advantageously be applied to the surfaces of a variety of medical implants such as orthopedic implants, dental implants, cardiovascular implants, and the like, to obtain the advantages of tissue ingrowth, tissue coating, cell attachment, or to promote bone ingrowth and superior fixation of the implant in the recipient's skeletal structure.
The conventional techniques for fabricating porous coatings on medical implants often produce two undesirable effects: small sections of the porous coatings are loosely adherent and coating particulates may become dislodged after implantation in the recipient's body; and loosely-held debris, produced in the coating process and remaining on the porous coating surface, separate from the surface and enter the recipient's body tissue after implantation of the implant. Similarly, loose or near-loose surface contaminants or metal particles may exist on grit-blasted, peened, or other types of roughened surfaces with similar consequences. The release of these debris and loosely adherent coating particulates into the body are cause for serious concern. Particulate debris produced by orthopedic implants can cause cell lysis and, in the case of for instance a hip stem prosthesis, stem abrasion, bone resorption and loosening of the implant. Further, detached debris may migrate into the spaces between articulating surfaces of a prosthetic joint and cause excessive wear of the joint thereby increasing the rate of generation of debris and aggravating adverse cell tissue responses, while reducing the useful life of the implant.
In order to minimize the amount of debris that a porous coated medical implant will release, conventional cleaning methods are currently employed. These include blasting the surface of the porous coated implant with particles of a hard material such as alumina, silicon carbide, steel shot, glass beads and the like, to remove loosely adherent coating particles and to dislodge other manufacturing debris. However, these cleaning methods produce "cross contamination" in that fragments of the hard particles used in the shot blasting cleaning process also adhere to the porous coated surfaces, which are by their very nature not smooth but which contain numerous tiny interstices into which fine particles may lodge. After implantation, the action of the recipient's body fluids and tissue and mechanical stresses exerted on the implant result in the release of this cross contamination debris into the body potentially leading to medically harmful effects.
What is needed is a method for cleaning medical implants which have undergone various surface roughening processes or that are coated with a porous coating that will not damage the coating, that will not cross contaminate the coating, and that will remove substantially all the manufacturing debris and any loosely adherent porous coating particles from the implant's surface.
U.S. Pat. No. 5,057,108 relates to a surface treatment process for stainless steel orthopedic implant devices. In this process, the stainless steel orthopedic implants are first blasted with stainless steel shot. This cold works the surface by introducing residual compressive stresses in the surface. This steel shot blasting treatment is followed by blasting with smaller sized glass beads, which is represented as improving the fatigue properties of the surface by working those areas not covered by the larger steel shot. Further, it is alleged that glass bead blasting helps clean the surface of any residual steel shot that may have been transferred to the target surface. After these two blasting steps, the surface is electropolished and then passivated by immersion in nitric acid solution to produce a protective oxide film on the finished part. Significantly, the patent does not address medical implants coated with a porous coating. Such a coating would become contaminated with fragments of stainless steel and glass bead shot. The subsequent electro-polishing and passivation treatments would not remove this cross contamination debris.
Likewise, East German patent application 133 914 relates to a process for cleaning continuous metal plate surfaces with a two-component medium. The medium includes (1) a carrier which can be gaseous carbon dioxide, sodium hydroxide solution, or other chemically active solutions; and (2) a granular solid selected from hard particles such as silicon carbide, glass beads, and the like. While the process uses granular solid hard particles which are hard enough that they do not stick to the surface being cleaned and thus do not leave large amounts of residue, these particles are not soluble in liquids used for cleaning and, thus, must leave microscopic residual particles on the surface being cleaned. This would be undesirable in medical implants since small particulate debris is the most potent initiator of adverse cell response. Furthermore, the patent application does not address the problems encountered in cleaning a roughened surface or porous-coated medical implant of surface debris and loosely held coating particulates.
Japan Laid Open 137 848 is directed to the "polishing" of surfaces with ice particles. It is represented that ice particles, due to their low density, will present a relatively low impact when blasted onto a metal surface for polishing and are therefore not very effective as a polishing medium. It is asserted that acid or alkali frozen solutions should be used as shot particles to achieve mechanical polishing, etching and degreasing of a surface at the same time. The treatment of porous-coated medical implants to remove surface debris or loosely held coating particulates is not addressed.
United Kingdom patent 1,397,102 is directed to the problem of cross contamination introduced onto a smooth surface when it is cleaned with an abrasive cleaner to prepare the surface for receiving a coating. The patent suggests using dry ice particles as shot to bombard the surface being prepared to receive a coating. The dry ice subsequently vaporizes leaving behind no cross contamination residue. West German patent application DE 37 20 992A1 likewise relates to a method for cleaning blast tool surfaces using dry ice particles. However, neither of these references address the cross contamination and biocompatibility problems associated with cleaning porous coated or roughened surface implants to avoid debris effects within the human body.
There yet exists a need for a process to clean medical implants supplied with a porous coating. The method should not damage the porous coating but should remove substantially all debris from the surface as well as any loosely-held particulates of the coating so that these will not separate from the implant in the recipient's body at a later time causing undesirable medical effects. Further, the method should not introduce cross contaminating debris into the implant surface but should provide a cleaned implant surface containing a reduced number and size of loose or potentially loose debris particles. Finally, the cleaning method should desirably provide a surface that is readily amenable for further treatment, such as passivation, and implantation into a recipient.